Walter Mondale, Minnesota's attorney general at the age of 32, transformed a ceremonial office into a center for activist government. In another epoch, suggests Yale historian Gillon in this brisk, hard-hitting political biography, the ambitious senator who helped shape national policy on education, welfare, civil rights and child care would have been a sure bet to win the presidency. Yet, in Gillon's appraisal, Mondale failed to develop a political strategy for reaching middle-class voters, and Reagan's gospel of individualism had far greater appeal to the electorate than the philosophy of shared responsibilities that guided Mondale's disastrous presidential bid in 1984. Gillon discloses that Mondale, frustrated as Carter's vice president, flirted with the idea of resigning in 1979. He credits Mondale as the major force in building a consensus among warring factions of the Democratic Party, but faults his increasingly irrelevant political vision. Photos. (Aug.)